The technical field of this invention is automatic control of motor vehicles.
The development of automatic control for motor vehicles is proceeding well beyond the simple, speed maintaining automatic throttle controls known popularly as xe2x80x9ccruise controlsxe2x80x9d. The automatic speed maintenance is being extended to lower speeds, even to stop and go operation; and such operation requires the ability to accurately and reliably sense when a vehicle is stopped.
Most vehicles manufactured today are equipped with one or more wheel speed sensors, due to their widespread use in anti-lock brake and traction control systems. A typical wheel speed sensor has a rotational member with a known number of teeth in a ring configuration. This member is attached to the wheel of the vehicle so that it rotates with the wheel. A stationary tooth sensor adjacent the member generates a pulse train signal from the detected passage of the teeth as the member rotates with the wheel. The speed of the wheel is calculated from the time duration T between pulses due to consecutive teeth:
xe2x80x83Wheel Speed=x/(y)(T),
where x is the longitudinal distance traveled per rotation of the member, y is the number of pulses (teeth) per rotation, and T is the time between pulses. Any combination of wheels can be used to calculate an estimate of the current vehicle speed.
But if the vehicle is stopped, the pulses cease; and the timebetween pulses becomes indefinite. Thus, absolute sensing of vehicle stop with such sensors is impossible, since it would require an infinite time for the detection. But by the selection of a threshold Tmax, the stationary determination can be made when the time between pulses exceeds the threshold. This is the same as limiting the low wheel speed and calling anything below a speed WSstop as indicating a stationary vehicle. For example, a stopped vehicle may be defined, with respect to a particular wheel speed sensor on the vehicle, as requiring no two consecutive signal pulses from the wheel speed sensor for a Tmax of about one half (0.5) second. But a problem occurs with such sensors if the specification for stop determination requires that it be performed within a shorter time period, for example one third (0.33) second. Even if no second pulse is received from the wheel speed sensor for one third of a second after a first pulse when the time allotted for determination has elapsed, there is no guarantee that it will not occur prior to one half second after the first pulse.
In addition, most wheel speed sensors used in vehicles for anti-lock braking and traction control tend to be less expensive passive wheel speed sensors rather than more expensive active wheel speed sensors. Passive wheel speed sensors provide output pulses generated solely by voltage induced in a coil by motion of the passing teeth of the sensor disk rotation with the wheel. The amplitude of pulses generated by such induction decreases with the speed of the motion. Thus, at the low wheel speeds at which stoppage of the vehicle is determined, the pulses generated by passive wheel speed sensors tend to be of significantly lower amplitude and have a more rounded, less distinct shape than those formed at the higher wheel speeds typical of anti-lock braking or traction control events, and this limits the accuracy of pulse edge detection at such low wheel speeds.
The apparatus and method of the invention provide a stopped vehicle determination within a predetermined maximum determination time, period, wherein a stopped vehicle is defined as a vehicle with forward speed lower than a predetermined minimum speed indicative of an essentially stopped vehicle and the vehicle has a plurality of wheels.
A rotational wheel speed sensor coupled to one of the plurality of wheels is of the type that provides a series of consecutive pulses having a time period therebetween varying inversely with vehicle forward speed, with the time period between pulses at the predetermined minimum speed indicative of an essentially stopped vehicle being greater than the predetermined maximum time period. A wheel speed determined stopped vehicle signal is generated when the time between pulses from the rotational wheel speed sensor is at least equal to the predetermined maximum determination period.
A range rate signal, relative to a detected object forward of the vehicle, is derived from a forward-looking radar unit; and a radar determined stopped vehicle signal is generated when the range rate signal from the forward-looking radar is less than a predetermined threshold. Finally, a combined stopped vehicle signal is generated when the wheel speed stopped vehicle signal and the radar stopped vehicle signal exist simultaneously.
The wheel speed determined stopped vehicle signal does not by itself guarantee a stopped vehicle, since it must be generated by the end of the maximum determination time, before the end of the period allowed for a second pulse from the wheel speed sensor. Likewise, the radar determined stopped vehicle signal does not by itself guarantee a stopped vehicle, since a forward object with an essentially zero range rate might be another vehicle proceeding forward at essentially the same speed. But when used together in an AND logic configuration, the radar determined stopped vehicle signal can provide the final degree of accuracy for high reliability to a wheel speed determined stopped vehicle signal that guarantees a forward vehicle speed at least close to zero.